The present invention relates to a sealable, transparent multilayer film comprising a base layer, essentially of polypropylene, which is coated on both sides with further polyolefin layers.
German Offenlegungsschrift No. 1,694,694 discloses heat-sealable laminates which consist of an oriented polypropylene film which possesses at least one heat-sealable layer made of an ethylene-propylene copolymer of 2 to 6% by weight of ethylene and 98 to 94% by weight of propylene. Though these films possess good heat-sealability, they are not as clear and scratch resistant as desired, and furthermore they have unsatisfactory processing characteristics on high-speed packaging machinery.
European Pat. No. 27,586 discloses sealable polypropylene films which have a sealing layer of an ethylene homopolymer or copolymer, provided with a long-chain aliphatic amine, an incompatible thermoplastic component and a polydialkylsiloxane. It is true that these films constitute an improvement compared to German Offenlegungsschrift No. 1,694,694, but they still have insufficiently reliable running characteristics on horizontal form-fill-seal machines.
German Offenlegungsschrift No. 2,941,140 discloses a packaging material which consists of a base layer of a propylene polymer and a surface layer of a blend of a propylene-ethylene copolymer and a (C.sub.4 -C.sub.10) -.alpha.-olefin-propylene copolymer. This surface layer can also contain a low molecular weight thermoplastic resin as well as silicone oils. Such packaging materials have the disadvantage that they are prone to scratching and still have unsatisfactory optical properties.
German Offenlegungsschrift No. 3,247,998 discloses a film which has sealing layers of a polyolefin copolymer or terpolymer of a particular composition and which, as a result of modification with a propylene homopolymer, a dimethylpolysiloxane and a synthetic resin, has not only good processing characteristics on high speed packaging machinery but also improved optical appearance. Although this film does meet the requirements made of a packaging film from the point of view of machine running, the handling of the film presents considerable problems due to the extremely smooth surfaces, which have very low coefficients of friction. Thus, on slitting these films down from the original roll width (4-6 m) to smaller widths, there is increased sideways drift, i.e., due to the sideways movement of the film at high speeds, and the desired width is not achieved exactly but exceeded to a greater or lesser degree. The term "making-up" will be used in the text which follows to include the slitting and winding-up process.
A further disadvantage of the film described above is its high tendency to telescope on the roll during handling, either on the slitting machine or on insertion into the packaging machine. The term "telescoping" denotes a sideways shift of the film on the roll, in which surfaces resting on one another slide to one side, having the effect that the edges of the film are no longer exactly superposed but are each staggered by a small amount in one direction so that the entire roll protrudes, on one side, to a greater or lesser extent beyond the winder core. Telescoping normally occurs if the rolls are knocked or kept slightly inclined.
Due to these difficulties, the machine speed during making-up must be reduced to about half its normal value and the subsequent handling of the rolls must be carried out with extreme care.
German Offenlegungsschrift No. 3,331,983 describes a sealable polyolefin multilayer film having good processing characteristics on high speed packaging machinery and at the the same time exhibiting good making-up characteristics and no tendency to telescope. However, the scratch resistance of this film still requires improvement.